Habitat Feature: Milkweeds

Milkweeds are an excellent choice for a habitat garden. They are a host plant for monarch butterflies and other specialized insects, and a nectar source for many pollinators. Their increasing scarcity in the wild makes them especially welcome in your garden, where patches of (ideally native-to-your-region) milkweeds can act as stepping stones for migrating monarchs; helping them maintain their population as each successive generation flits from backyard to backyard on their journey north and then south again. Below we explain what happened to all the milkweeds in the wild, discuss why they are critical for monarchs, advise on how to choose the correct species of milkweed for your region, and how to join the effort to collect and share seeds.

Did you know there is more than one kind of milkweed? At the big box stores you may find seed or plants of tropical milkweed (Asclepias curassavica) (pictured above), but this is not native to most of N. America (a handful of places in California, Texas, Louisiana, and Florida are the exceptions). Whether or not this tropical species provides a net benefit or loss to the monarch population is a topic of open research.

Over the summer of 2015, Karen Oberhauser from the University of Minnesota Monarch Lab did a Q&A for folks with questions about monarchs. She says this of the Asclepias curassavica controversy:

Tropical milkweed (Asclepias curassavica) is not native to the United States or Canada. Because it is attractive and easy to grow, it is often the most widely available milkweed at commercial nurseries. Because tropical milkweed historically occurs in the New World tropics, it is adapted to grow year-round, whereas most native North American milkweed species die back each winter. When tropical milkweed is planted in the coastal southern U.S. and California, these plants continue to flower and produce new leaves throughout the fall and winter, except during rare freeze events. Potential negative effects on monarchs include 1) continuous breeding on the same plants, which can lead to a build-up of Ophryocystis elektroscirrha (OE) infection, and 2) availability of milkweed during a time that it is not naturally available, and so potential consequent impacts on monarch breeding during the fall migration.sourceopen_in_new

As her research continues, we will keep our audience posted on her results. Until then, consider a milkweed native to your region which will not require extra water, and will cycle according to the climate (see below for ideas), or cut back your tropical milkweed in the winter months to mitigate build-up of OE spores and stop monarch breeding during fall migration.

Historically, milkweed has been a common plant throughout the vast prairie regions in the United States. Twenty-nine species of Asclepias, most of them grassland species, are native to the late summer breeding range of the monarch. In all of North America there are over 100 species of milkweeds, showing amazing diversity. Check-out our slide show at the end of this article for a sampling of these colorful, interesting plants.

As the prairies came under cultivation there was a corresponding increase in the distribution and abundance of Common Milkweed (A. syriaca), which was able to live in and around the corn and soybean crops dominating prairie agricultureopen_in_new.

As a result, common milkweed, A. syriaca, has been the primary food of monarch butterflies migrating across the agricultural regions. In a sample of 394 overwintering monarch butterflies in Mexico, 382 had fed on the common milkweed as larvaeopen_in_new. This important host species, however, has been losing ground as Roundup-ready corn and soybean have been replacing traditional crops, allowing widespread use of herbicides that target all weeds in agricultural fields, including milkweeds. The resulting decrease in milkweed availability likely accounts for the dramatic decline in monarch populations witnessed over the last 20 years. It is estimated that since 1996, we’ve lost enough milkweed habitat to cover an area the size of Texas (~69 million hectacres)open_in_new.

Milkweed is named for its milky sap, which consists of latex-containing alkaloids and several other complex compounds including cardenolides. In the image above, showing a California ladybug larva (not a monarch caterpillar!), the white spots are places where the milkweed’s “milk,” or latex-containing sap, has leaked out as a result of aphid damage. This ladybug larva has likely consumed most of the aphids, and is now preparing to pupate on the underside of the milkweed leaf.

Most species of milkweed, when ingested, are toxic to animals (vertebrates specifically) due to the cardenolides, plant alkaloids that are also called cardiac glycosides because they can disrupt vertebrate heart function. When caterpillars eat the leaves of the milkweed, they also ingest the plants’ toxins. There is a lot of variation in the amount of cardiac glycosides in different species of milkweeds. As caterpillars eat, these toxic compounds build-up in their bodies, and eventually are found in high concentrations in the wings and exoskeletons of adult butterfliesopen_in_new, making the insects themselves toxic to predators. Caterpillars consume a lot of milkweed leaves in order to gain enough mass in such a short time. Notice, in the image above, the vast difference in sizes between the young caterpillars and those that are just 10 days older. It is estimated that a single monarch caterpillar needs around 20 entire leaves to grow large enough to successfully pupate.

Birds and Monarchs

Did You Know? The story of birds and monarchs is more complicated than you may have learned in school? You’ve probably heard the narrative of how birds learn to avoid brightly-colored monarchs because they taste bitter. In the late 50s Dr. Brower (1958) used captive Florida Scrub-Jays (Aphelocoma coerulescens) to test this hypothesis. Some had monarchs and swallowtails put in their cages, while others were offered viceroys and swallowtails. One group of birds ate all of the swallowtails offered and half of the viceroys. The other group ate all of their swallowtails and not a single monarch. Dr. Brower concluded that monarchs are inedible, and that the viceroy is partially protected by its resemblance to the monarch.

The Plot Thickens. Suspecting that the story was more complex, follow-up researchers engaged in a more naturalistic (with wild, rather than caged birds) observational study, because they felt that captive birds do not always behave the way wilds ones do. Here is how they summarize their research:

In the summer of 1963, five species were frequent visitors; cardinals, brown thrashers, grackles, robins, and english sparrows. They were fed on table scraps of every sort, birdseed, and suet. A white enamel pan was placed on the ground in this yard between the birdbath and one of the feeders. Live (and lively) monarch butterflies, after having their wings trimmed… were placed in the pan every morning before dawn. …and they did elicit a feeding response in the birds. Nearly every morning for two weeks the birds emptied the pan. The best customers were the brown thrashers, one pair of which was observed feeding the butterflies to their young. Between July 18 and July 31, 110 of 112 … monarchs were eaten. The birds that ate them could have lived off a bounteous Iowa summer, or the food in the bird feeders if monarchs were distasteful to them.sourceopen_in_new

In overwintering grounds in Mexico, two species of birds are well-known for preying on monarchs: Black-backed Orioles and Black-headed Grosbeaksopen_in_new. At overwintering sites in California researchers have observed Chestnut-backed Chickadees (Poecile rufescens), European Starlings (Sturnus vulgaris), Western Scrub-Jays (Aphelocoma californica), and California Towhee (Pipilo crissalis) eating monarchs. All of these birds have different strategies for coping with the toxic compounds in their bodies, ranging from eating the parts of the insect with less toxins, to taking extended feeding breaks.

Milkweeds are also important as a source of nectar to many pollinators. In the image above, see how many swallowtails you can spot (most are spicebush swallowtails) on this lovely milkweed commonly called Butterfly weed (A. tuberosa). By planting milkweeds, you aren’t just providing a host plant for monarchs, but a great pollinator plant in general.

Milkweeds are perennial plants, living for several years and regrowing each spring from rootstock, not just from seed, like annuals. The young, regrowing milkweeds in the image to the left are strong robust plants even early in the Spring, because of their perennial habit. These plants (likely growing from the same colonial restock) are clones. Both will grow-up to produce seed late in the Fall.

If you want to help with the declining monarch population, you can do several things. Plant milkweeds, harvest and share milkweed seeds, advocate that ditches and medians not be mowed regularly (they are important wild spaces for milkweed), and refrain from using pesticides in your yard. Check out this cool species map to find your region’s milkweeds. Then use the Milkweed Market to find the appropriate kind of milkweed seed for your area.

Harvest Milkweed Seed

Harvesting milkweed seed, either for personal use or to donate to a seed-saving service, is an excellent way to help. Check-out this wonderful seed-harvesting tutorial over at Monarch Butterfly Garden to learn how. Late fall is a great time to get out there and harvest. To donate your seeds, or request seeds native to your region to plant on your own property, check out Monarch Watch’s donation program. It is a really neat way people like you are making native seeds accessible.